Aluminum metal is produced industrially by electrolysis of smelter grade (or other) alumina in a molten electrolyte using the well-known Hall-Héroult process. This process may be referred to herein generally as a smelting process. The electrolyte is contained in a pot comprising a steel pot shell, which is coated on the inside with refractory and insulating materials, and a cathodic assembly located at the bottom. Carbon anodes extend into the electrolyte, which comprises molten cryolyte and dissolved alumina. A direct current, which may reach values of more than 500 kA, flows through the anodes and the electrolyte to generate chemical reactions that reduce the alumina to an aluminum metal, and that heat the electrolyte by the Joule effect to a temperature of approximately 960° C. Emissions from the electrolytic cell comprise a number of gaseous and particulate constituents, also referred to as process gases, such as hydrogen fluoride (Fg) and particulate fluoride (Fp).
Dry adsorption and chemisorption of gaseous fluorides onto the surface of fresh alumina, followed by the recycle of the fluorinated alumina back to the electrolytic cell as the feed material for an aluminum smelting process, is widely accepted as the best available technique for abating fluoride emissions from an electrolytic cell. An injection type dry scrubbing system uses adsorption followed by chemisorption of gaseous hydrogen fluoride onto the surface of smelter grade alumina, and then filters the alumina and particulate before releasing scrubbed gases (including residual emissions) to the environment. Depending on the electrolytic cell operating current and operating conditions (i.e. ventilation rate, electrical resistance, which varies with the anode to cathode distance (ACD), and electrolysis current), the temperature of the process gases exhausted from conventional electrolytic cells typically varies between 100° C. to 140° C. above ambient temperature. Because the temperature of the process gas exhausted from the electrolytic cell varies indirectly with the moist ambient air flow entering the electrolytic cell, conventional smelting process systems with significantly reduced ventilation flow can theoretically generate process gas temperatures up to about 400° C.
The conventional smelting process is inherently inefficient with an energy to metal conversion efficiency of just 50%. The balance of the energy is lost to the environment in the form of low grade waste heat. Because the current amperage in an electrolytic cell has and will continue to exceed 500 kA, the energy released to the process gases has and will continue to increase the process gas exhaust temperature. The adsorption efficiency of gaseous fluoride on the surface of the alumina will thus, be reduced if suitable countermeasures to cool the process gases are not implemented by conventional injection type dry scrubbing systems.
The electrolytic cell is generally controlled to maintain a preferred thermal equilibrium—meaning heat dissipated by the electrolytic cell is balanced by the heat produced in the cell. The point of a preferred thermal equilibrium is that which achieves the most favorable operating conditions in not only technical, but economic terms. Maintaining an optimal electrolyte temperature, for example, represents an appreciable saving on the production cost of aluminum due to the reduced energy consumption by the electrolytic cell. Maintaining the preferred thermal equilibrium depends largely on the physical design parameters of the electrolytic cell such as the dimensions and properties of the cathode side wall lining, the cover material (crust) granulometry/thickness, and the operating conditions (e.g. electrolysis current). The electrolysis current amperage may be modulated, for example, under different operating conditions depending on the electrical grid supply and demand. Modulating the current has a direct effect on the heat flux along the electrolytic cell sidewalls, which varies along the vertical surface. Peak heat flux typically occurs at the molten electrolyte—molten metal interface where the electrical ohmic resistance (and resulting heat generation) is greatest between the anodes and cathode bars. Maintaining the preferred thermal equilibrium therefore, also depends on the ability to control heat loss from a preferred area of the electrolytic cell side walls during different current amperages in the electrolytic cell.
Current technology to control heat loss from the electrolytic cell includes heat exchangers and forced cooling systems, which use fixed, non-adjustable, elements such as nozzles and heat exchangers to enhance cooling of the electrolytic cell side walls. These technologies are capable of modulating (increasing or decreasing) the total heat loss from the side walls as the current amperage flowing through the electrolytic cell is modulated upward when relatively low-cost power is available from an electrical grid and downward to conserve power during periods of peak demand on an electrical grid. These technologies, however, are not capable of adjusting modulated cooling within a preferred area of the electrolytic cell side walls based on operating conditions in the electrolytic cell. In addition, these technologies direct the waste heat away from the electrolytic cell into the pot-room where the energy is lost to the environment in the form of low grade waste heat. As a result, these technologies can expose operating personnel to dissipated heat and entrained dust.